nix flake metadata: Show flake fingerprint

This is useful for testing/debugging and maybe for sharing eval caches
(since it tells you what file in ~/.cache/nix/eval-cache-v5 to copy).

(cherry picked from commit 1ff186fc6e)

tests/nixos/tarball-flakes.nix

@@ -66,6 +66,9 @@ in
     # Check that we got redirected to the immutable URL.
     assert info["locked"]["url"] == "http://localhost/stable/${nixpkgs.rev}.tar.gz"
 
+    # Check that we got a fingerprint for caching.
+    assert info["fingerprint"]
+
     # Check that we got the rev and revCount attributes.
     assert info["revision"] == "${nixpkgs.rev}"
     assert info["revCount"] == 1234